1. Field of the Invention
The present invention relates to a magnetic refrigerator that transmits heat by solid thermal conduction.
2. Description of the Related Art
Certain magnetic segments are known to exhibit significant temperature changes during magnetization or demagnetization. That is to say, changing an applied field in a heat insulation state changes the temperature of a magnetic material segment. This phenomenon is called a magnetic heat quantity effect. In a physical sense, the degree of freedom of magnetic spins inside the magnetic material segment is changed by the magnetic field. This in turn changes the entropy of a magnetic spin system (electron system responsible for magnetism). The entropy change instantaneously transfers energy between the electron system and a lattice system. This changes the temperature of the magnetic material segment. Magnetic refrigeration utilizes the magnetic heat quantity effect to perform refrigerating operations.
A gas compressing cycle is used for most refrigerating techniques in an ordinary temperature zone closely related to human daily lives, for example, refrigerators, freezers, and interior cooling. However, the refrigerating technique based on the gas compressing cycle has the significant disadvantage of emitting a particular chlorofluorocarbon gas to an environment to destroy an ozone layer. A hydrochlorofluorocarbon gas may also affect the environment in connection with global warming. Under these circumstances, a clean and efficient refrigerating technique has been desired to be put to practical use; such a refrigerating technique needs to avoid environmental destruction associated with the disposal of operating gas.
In recent years, magnetic refrigeration has been expected as an environment-oriented and efficient refrigerating technique. Much effort has been made to research and develop magnetic refrigerating techniques for the ordinary temperature zone.
As a useful refrigerating scheme for magnetic refrigeration in the ordinary temperature zone, Barclay in U.S. has proposed an AMR scheme (Active-Magnetic Regenerative Refrigeration) such as the one disclosed in, for example, U.S. Pat. No. 4,332,135. This method has been developed in view of the problem that “in a high temperature zone such as the room temperature, the lattice entropy of thermal disturbance increases relative to the entropy of a magnetic spin system, reducing a change in the temperature of a magnetic material segment caused by the magnetic heat quantity effect”. This method rather actively utilizes the lattice entropy considered to be an inhibiting factor for the magnetic refrigeration in this temperature zone. Specifically, this refrigeration scheme allows the magnetic material segment to perform a magnetic refrigerating operation based on the magnetic heat quantity effect and to exert a thermal storage effect to store cooling heat generated as a result of the magnetic refrigerating operation.
The AMR scheme is characterized by controlling heat transfer to and from the magnetic refrigerating material so as to combine the magnetic refrigerating operation based on the magnetic heat quantity effect with the thermal storage effect to form an advantageous temperature gradient over the entire magnetic refrigerating material. A specific method involves, for example, dividing a solid magnetic refrigerating material into small pieces and arranging the pieces so as to suppress the direct heat transfer among the pieces. Then, cooling heat generated by the magnetic heat quantity effect of each small piece is transferred from the small piece to a heat transporting medium (liquid or gas) by heat exchange. The heat transporting medium is allowed to flow to transport the cooling heat generated. This transportation requires a channel for the heat transporting medium as well as sufficient heat exchange between the magnetic refrigerating material and the heat transporting medium. The magnetic refrigerating material thus needs to be shaped so as to have a large specific surface area and to provide a channel for the heat transporting medium. The actual shape may be a honeycomb structure obtained by forming a material into a thin plate and folding the thin plate into bellows, laminated meshes, or spherical particles filled into a container.
Zimm Gschneidner Pecharsky et al. in U.S. has actually made an experimental AMR-based magnetic refrigerator using a superconductive magnet, to successfully execute a continuous and steady magnetic refrigerating cycle in the room temperature zone (1997). The AMR-based magnetic refrigerator uses Gd (gadolinium) as a magnetic refrigerating material. Gd (gadolinium) is formed into fine spheres of diameter about 0.3 mm, which are then filled into a magnetic refrigeration working chamber. With this magnetic refrigerator, Gd spherical particles, a magnetic refrigerating material, contact one another at points. This sharply reduces the heat transfer among the spherical particles. On the other hand, in the magnetic refrigerator, an inlet and outlet for the heat transporting refrigerant are connected to the respective ends of the magnetic refrigeration working chamber to provide a structure that allows the heat transporting refrigerant to be guided into and out of the working chamber. The heat transporting medium is water or a mixed solution of water and ethanol. A coil-like superconductive magnet is provided outside the magnetic refrigeration working chamber. The magnetic refrigeration working chamber is movable. The magnitude of a magnetic field applied to the magnetic refrigerating material can be changed by displacing the magnetic refrigeration working chamber from a bore space in the coil along the axis of the superconductive coil; the magnetic refrigerating material is housed inside the magnetic refrigeration working chamber. Here, AMR is executed in steps described below. First, (A) the magnetic refrigeration working chamber is moved to the bore space in the superconductive magnet, and a magnetic field is applied to the magnetic refrigerating material. (B) The heat transporting refrigerant is then moved (allowed to flow) from one end to the other end of the magnetic refrigeration working chamber to transport heat. (C) The magnetic refrigeration working chamber is then moved from the bore space in the superconductive magnetic to the outside, and the magnetic field applied to the magnetic refrigerating material is removed. (D) The heat transporting refrigerant is then moved (allowed to flow) from one end to the other end of the magnetic refrigeration working chamber (direction opposite to that for the refrigerant movement in B) to transport heat. Repetition of the thermal cycle steps (A) to (D) raises the temperature of the magnetic refrigerating material particles inside the magnetic refrigeration working chamber in association with the application of the magnetic field. Then, heat is exchanged between the particles and the heat transporting refrigerant, which then moves forward. Heat is then exchanged between the heat transporting refrigerant and the particles. The subsequent removal of the magnetic field lowers the temperature of the magnetic refrigerating material particles. Heat is then exchanged between the particles and the heat transporting medium, which then moves backward. Heat is then exchanged between the heat transporting refrigerant and the particles. That is to say, the heat transfer between the spherical particles, a magnetic refrigerating material, is predominated by indirect heat conduction via the heat transporting refrigerant. Direct heat conduction based on the contact between the spherical particles is suppressed. Each spherical particle stores heat owing to a high thermal storage effect. Thus, repetition of the heat cycle varies the temperatures of the adjacent spherical particles, resulting in a temperature gradient in the direction of a heat flow of the heat transporting medium. This makes it possible to make a great difference between the temperatures at the opposite ends of the magnetic refrigeration working chamber in a steady state.
In the example of the magnetic refrigerator proposed by Zimm et al., a superconductive magnet was used to change the magnetic field from 0 to 5 Tesla in the room temperature zone to successfully achieve refrigeration with a refrigerating temperature difference ΔT=about 30° C. between the opposite ends of the magnetic refrigeration working chamber. It has also been reported that a refrigerating temperature difference ΔT of 13° C. lead to a very high refrigerating efficiency (COP=15; excluding power input to a magnetic field generating unit). Domestic refrigerators based on a conventional compression cycle with chlorofluorocarbon have a refrigerating efficiency (COP) of about 1 to 3.
However, the AMR-based magnetic refrigerator disclosed in U.S. Pat. No. 4,332,135 uses a superconductive magnet to apply a magnetic field of about 2 to 5 Tesla to a magnetic refrigeration operating substance as described above. Thus, the superconductive magnetic requires a cryogenic environment at about 10 k. However, such a cryogenic environment requires a freezing mixture such as liquid helium, or a cryogenic refrigerator. This complicates the magnetic refrigerating system and increases its size and cost. Instead of the superconductive magnetic, an electromagnet may be utilized as a unit that generates a magnetic field. However, even the electromagnet requires a large current to be input in order to generate a large magnetic field of about 1 Tesla. This makes the system complicated and inconvenient, for example, requires water cooling in order to reduce Joule heat. Further, running costs rise and the size of the system increases as is the case with the superconductive magnet. The system thus becomes expensive.
On the other hand, the above embodiment of the AMR scheme forms a magnetic refrigerating material into spherical particles and fills a container with the particles. The embodiment uses a mixed solution of water and ethanol as a heat transporting medium. That is to say, the transportation of cooling heat does not depend on the direct heat conduction between solids but on the heat exchange between a solid and a fluid (in this case, a liquid) followed by fluid movement. Thus, not only a solid but also a fluid such as a liquid or gas is required as magnetic refrigerating materials for a magnetic refrigerating operation. Moreover, a driving mechanism is required to move the fluid. This economically disadvantageously increases the scale of the system.